Recently, powder coating has been widely used for general metal coating from the viewpoint of advantages such as pollution-free, resource saving, and power saving, and particularly, it is widely used for road/construction materials such as bridges, bridge railings, gates, fences, house siding materials which are required to have weather resistance; bodies and parts of automobiles; and home electric appliances.
As the powder coating, a method of coating a thermoplastic fluororesin powder made of an ethylene/tetrafluoroethylene copolymer is known (the following Patent Documents 1 to 3).
Further, as a thermosetting powder coating material to be used for the powder coating, a polyester resin powder coating material containing a block isocyanate compound, triglycidyl isocyanurate, etc., as a curing agent, or an acrylic resin powder coating material having a glycidyl group and using a dibasic acid as a curing agent, is known (the following Non-Patent Document 1).
The coating material made of the above thermoplastic fluororesin powder has suitable weather resistance, but it has drawbacks in coating workability such that the dispersibility of a pigment, glossiness of the coated film surface and adhesion with a base material are poor, and in the coating operation, it is required to be heated to a temperature higher than the melting point of the fluororesin, whereby a large amount of heat energy is consumed.
On the other hand, with respect to the latter polyester resin powder coating material or acrylic resin powder coating material, instead of having the same drawbacks as the above powder coating material made of the thermoplastic fluororesin powder, it has a drawback of not having sufficient weather resistance.
To overcome such a problem, the following Patent Document 4 suggests a thermosetting powder coating composition which comprises (A) a fluorinated copolymer containing fluoroolefin units and having a fluorine content of at least 10 wt %, an intrinsic viscosity in a range of from 0.05 to 2 dl/g, as measured in tetrahydrofuran at 30° C., a glass transition temperature of from 30 to 120° C., and crosslinkable groups, and (B) a curing agent (which is one except for a polyfunctional organic silicon compound containing an isocyanate group directly bonded to a silicon atom) which may react with the crosslinkable groups contained in (A) the fluorinated copolymer to form crosslinks, in a weight ratio of (A)/(B) being from 40/60 to 98/2, wherein the loss on heat of (A) the fluorinated copolymer is at most 5 wt % under a condition of 105° C. for 3 hours; and particularly such a thermosetting fluororesin powder coating composition having a glass transition temperature of from 30 to 120° C.
However, recently, as the application range of the thermosetting fluororesin powder coating composition has expanded to outer plates of buildings or aluminum sashes, a coated film is required to have a higher level of flexibility and appearance.
The flexibility of the coated film, for example, can be improved by increasing the molecular weight of the fluorinated copolymer which will be a binder component in the coating composition. However, such means an increase of the melt viscosity of the resin, and with a powder coating material to be subjected to a film-forming process covering from melting by heating to a curing reaction, the appearance of a coated film will be poor if the melt viscosity increases. On the other hand, if the molecular weight of the fluorinated copolymer is decreased, the melt viscosity becomes low, and although the appearance as the coated film will be improved, the flexibility will decrease.
Further, even if the molecular weight of the fluorinated copolymer is relatively large, it is possible to lower the melt starting temperature by lowering the glass transition point, whereby it is possible to satisfy both physical property and appearance by sufficiently spreading the composition before the reaction initiation temperature of the curing agent. However, if such a method is actually used, there will be a problem of blocking such that the glass transition point will be at most 50° C., and the solid powder coating material will be hardened during its storage and will not be applied.
Thus, it is extremely difficult to satisfy both the appearance and flexibility of the coated film while preventing the blocking in the powder coating material.
To overcome such a problem, the following Patent Document 5 suggests a composition made by mixing a fluorinated copolymer having vinylidene fluoride as the main component and an acrylic polymer having methyl methacrylate as the main component. However, the copolymer having vinylidene fluoride as the main component has crystallinity, whereby the transparency tends to decrease, and it is difficult to form a coated film excellent in appearance.
Further, the following Patent Document 6 introduces a technique of blending a thermoplastic fluororesin in a thermosetting fluororesin powder coating composition. According to such a method, it is possible to expect improvement of the physical property of the thermoplastic fluororesin having a relatively high molecular weight and improvement of the appearance without causing a curing reaction as the thermoplastic resin, but the compatibility between the thermosetting fluororesin powder coating composition and the thermoplastic fluororesin will be poor and the gloss tends to deteriorate, whereby it is difficult to obtain a coated film excellent in the appearance. Further, when the thermoplastic fluororesin is to be blended in the thermosetting fluororesin powder coating composition, there will be a practical difficulty in production. That is, with respect to a melt-kneading method which is currently main as a process for producing a thermosetting powder coating composition, it is necessary to pulverize thermosetting powder coating pellets after the kneading step, and in order to efficiently pulverize the thermoplastic resin, freeze pulverization such as cooling down to a stiffening point or lower, is required, whereby the pulverization will require a great amount of energy.
Patent Document 1: JP-A-61-181567
Patent Document 2: JP-A-61-181571
Patent Document 3: JP-A-61-181572
Patent Document 4: Japanese Patent No. 1973174
Patent Document 5: JP-A-9-165535
Patent Document 6: JP-A-2004-35876
Non-Patent Document 1: “Powder and Industry” Compiled by The Association of Powder Process Industry and Engineering, Japan, 1984, February issue, p. 33 to 42